Aluminum alloy

ABSTRACT

This disclosure teaches a novel aluminum alloy having high resistance to oxidation during hot rolling, improved surface appearance and improved bright anodizing characteristics and the method of processing same. The alloy contains from 0.5 to 3 percent magnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percent iron, from 0.001 to 0.15 percent silicon, balance essentially aluminum, wherein the silver is substantially dissolved in solid solution in the matrix.

rock et a1.

[ ALUMINUM ALLOY [75] Inventors: Andrew J. Brock, Cheshire; Michael J. Pryor, Woodbridge, both of Conn.

[73] Assignee: 61in Corporation, New Haven,

Conn.

22 Filed: June 1,1971

[21] Appl. No.: 148,582

[52] U.S. Cl. ..75/147, 75/141, 75/142, 75/146 51 1m. Cl ..C22c 21/02 [58] Field of Search ..75/l47, 146, 141, 142; 148/32, 148/325 [56] References Cited UNITED STATES PATENTS 2,586,647 2/1952 Gresham et al. ..75/147 ]March 13, 1973 3,306,717 2/1967 Lindstrandetal. ..75/147 Primary Examiner-Richard 0. Dean Attorney-Robert H; Bachman [57] ABSTRACT silver is substantially dissolved in solid solution in the matrix.

3 Claims, 2 Drawing Figures ALLOY -A OX/DAT/ON TIME HOURS ALLOY-B ALUMINUM ALLOY BACKGROUND OF THE INVENTION Alloys containing from 0.5 to 3 percent magnesium with small amounts of iron and silicon find extensive use commercially, for example, particularly as bright anodized automotive trim. They are generally fabricated to sheet, bright dipped, anodized in sulphuric acid and sealed so as to expose a bright reflective corrosion resistant surface for decorative and functional purposes. In automotive applications these bright anodized aluminum alloys have appearance somewhat similar to buffed stainless steels or to chrome-plated brass but are much more economical to the user.

One of the problems in processing aluminum-magnesium alloys for bright anodizing is the necessity of retaining magnesium-silicide in solution during the hot rolling process. This is mandatory in order to obtain proper response to bright dipping and to sulphuric acid anodizing so that excellent specular reflectivity is obtained. Retaining magnesium-silicide in solution during hot rolling involves initiating the rolling at unusually high temperature in the range of 850 to 975 F depending upon the magnesium content of the alloy. At these high temperatures, the oxidation resistance of aluminum-magnesium alloys deteriorates rapidly and copious amounts of loosely adherent magnesium oxide are formed as part of the high temperature oxidation product during hot rolling. The magnesium oxide can either adhere to the rolls or can be ground into the soft metal surface. Either eventuality gives rise to surface defects known generically as hot' mill pickup. This pickup is never completely removed during subsequent processing and results in defects in the final bright anodized sheet that tend to detract from specular reflectivity and from subsequent corrosion resistance.

Accordingly, it is a principal object of the present invention to improve the high temperature oxidation resistance of aluminum-magnesium alloys intended for use in the bright anodized condition.

It is an additional object of the present invention to provide novel aluminum alloys having a superior response to bright dipping and a method for processing same which obtains improved properties.

It is a further object of the present invention to provide novel aluminum alloys which have improved reflectivity after bright anodizing.

Further objects and advantages of the present invention will appear from the ensuing specification.

SUMMARY OF THE INVENTION In accordance with the present invention, the foregoing objects can be readily achieved.

The improved alloys of the present invention consist essentially of from 0.5 to 3 percent magnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percent iron, from 0.001 to 0.15 percent silicon, balance essentially aluminum, wherein the silver is substantially dissolved in solid solution in the matrix.

The alloys of the present invention are characterized by many desirableadvantages. The alloys have markedly improved resistance to oxidation in the temperature range of 850 to 975 F which results in improved surface appearance after hot rolling. The alloys are tolerant to a broader range of solution composition in which they can be bright dipped. The alloys further exhibit substantially improved brightness after anodizing in sulphuric acid and sealing.

The present invention also provides a process for obtaining improved, bright anodizing alloys which comprises:

A. providing an alloy consisting essentially of from 0.5 to 3 percent magnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percent iron, from 0.001 to 0.15 percent silicon, balance essentially aluminum;

B. holding said alloy for at least 15 minutes within the temperature range of 850 to 975 F;

C. hot rolling said alloy with a starting temperature within said holding temperature range;

D. cold rolling said alloy; and

E. heating said alloy within the temperature range of 350 to 750 F for from 5 seconds to 8 hours.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings which form a part of the present specification:

FIG. 1 is a graph of weight gain versus time when a comparative Alloy A is compared with Alloy B, an alloy of the present invention. The graph and results are discussed in the examples.

FIG. 2 shows a plot of comparative Alloy A and Alloy B of the present invention showing specimen potential versus nitric acid concentration in mixed nitric acid-phosphoric acid bright dipping solutions. The graph and results are discussed in the examples.

DETAILED DESCRIPTION OF THE INVENTION As indicated herein and above, the alloys of the present invention are characterized by greatly improved resistance to oxidation within the temperature range of 850 to 975 F and exhibit 'a much superior hot rolled surface and specular reflectivity. after bright dipping. These improved surface characteristics are obtained in accordance with the alloys and process of the present invention. In addition, the alloys of the present invention have good mechanical properties and workability.

The favorable surface characteristics achieved by the alloys of the present invention result in greatly reduced product rejections due to hot mill pickup during hot rolling at high temperature. The oxidation problem cannot be solved in conventional alloys by reducing hot rolling temperature and thereby increasing oxidation resistance because magnesium-silicide now forms in the microstructure of the alloy and detracts from the response to bright dipping and bright anodizing.

The alloys of the present invention do not completely inhibit the formation ofmagnesium oxide as a reaction product during heating in the temperature range from 850 to 975 F. However, the amounts of magnesium oxide formed are tremendously reduced and it appears to exist as a compact transparent film that is not removed from the metal during hot rolling. It is a surprising feature of the present invention that the range of acid concentrations over which excellent brightening can be obtained in bright dipping is substantially increased with the alloys of the present invention. Accordingly, the incidence of unacceptable products in subsequent bright dipping and anodizing is significantly reduced. It has been found that due to the foregoing and due to the improved rolled surface, loss of brightness during anodizing in sulphuric acid is dramatically reduced.

As indicated hereinabove, the alloys contain from 0.5 to 3 percent magnesium, from 0.02 to 0.5 percent silver, from 0.001 to 0.2 percent iron, from 0.001 to 0.15 percent silicon, balance essentially aluminum. it is preferred that the magnesium content be from 0.8 to 2.8 percent and that the silver be from 0.02 to 0.1 percent. The alloy may also contain one or more of the following: manganese in an amount up to 0.10 percent, copper in an amount up to 0.10 percent, zinc in an amount up to 0.10 percent, chromium in an amount up to 0.05 percent, and titanium in an amount up to 0.05 percent. Naturally, amounts as low as 0.001 percent of the foregoing may be present.

The process of the present invention is briefly described hereinabove. The alloy is held within the temperature range 850 to 975 F for at least 15 minutes, with the maximum holding time not being critical. Following the holding step, the alloy is hot rolled, with the starting temperature being within said temperature range and with a preferred reduction of at least 70 percent. The alloy may be cooled from hot rolling by any desired means, such as by water quenching. The alloy is then cold rolled, with a preferred reduction of at least 50 percent followed by heating for from 5 seconds to 8 hours within the temperature range of 350 to 750 F.

As indicated hereinabove, the resultant material has been found to have many, highly desirable characteristics, such as improved response to bright dipping and bright anodizing and reduced oxidation rate.

The present invention will be more readily apparent from a consideration of the following examples:

EXAMPLE I A comparative Alloy A, containing 2.4 percent magnesium, 0.036 percent iron, 0.044 percent silicon, 0.041 percent copper, 0.0088 percent titanium, balance essentially aluminum, was cast as a direct chill ingot measuring 3 X 7 X 42 inches. One-eighth inch was milled off each face of the ingot. The ingot was preheated to 925 F and held at temperature for a period of 8 hours followed by rolling on polished steel rolls with two successive reductions in thickness of 20 and 30 percent. A substantial amount of oxide was transferred to the polished steel rolls and the surface of the hot rolled alloy was rough and discontinuous.

EXAMPLE II An alloy, Alloy B, containing 2.5 percent magnesium, 0.05 percent silver, 0.036 percent iron, 0.043 percent silicon, 0.045 percent copper, 0.0089 percent titanium, balance essentially aluminum, was direct chill cast, scalped and hot rolled in an identical fashion to that described in Example I. The polished steel rolls were essentially free from oxide transferred from the hot aluminum alloy and the hot rolled surface was uniform and free from defects.

EXAM PLE Ill Alloys A and B of Examples l and II were heated to 925 F, held for 8 hours and hot rolled in 10 passes to a final thickness of 0.1 inch. The temperature after the last pass was 700 F and the alloys were then quenched in still water. The alloys were subsequently cold rolled to a thickness of 0.030 inch and were partially annealed at 500 F for 2.5 hours. After processing, the surface of Alloy A showed extensive hot mill pickup of magnesium oxide. On the other hand, the alloy of the present invention, Alloy B, showed excellent surface smoothness and was substantially free from hot mill pickup.

EXAMPLE lV Alloys A and B, processed in accordance with Example III, were heated to 932 F in air and held at that temperature for 24 hours during which time continuous recording of the oxidation weight gain were made with a recording microbalance. The results are shown in FIG. 1. The graph clearly shows that Alloy B, the alloy of the present invention, had a much reduced oxidation rate and showed essentially no oxidation between times of around 4 and 12 hours. By contrast, the comparative Alloy A showed essentially a high and constant rate of oxidation during that period, with the weight gain curve being essentially linear.

EXAMPLE V Alloys A and B, processed in accordance with Example III, were immersed for 120 seconds in a solution containing percent phosphoric acid and various concentrations of an aqueous, 70 percent nitric acid solution with the specimen potentials being also measured. The temperature of the bath was 83 C. The results, summarized in FIG. 2, show that acceptable bright dipping was obtained for Alloy B, the alloy of the present invention, over a wider range of nitric acid concentrations than the comparative Alloy A. The reflectivity of Alloy B, the alloy of the present invention, after bright dipping was 79 percent with a silver mirror being used as a standard of percent reflectivity. On the other hand, Alloy A had a reflectivity of only 58 percent after bright dipping.

EXAMPLE VI The bright dipped specimens of Example V were anodized for 20 minutes in 15 percent sulphuric acid at 25 C with a current density of 20 amps per square foot. After anodizing the alloys were rinsed and were sealed for 10 minutes in boiling water containing 3 ppm phosphate added as Na HPO,. The reflectivity of Alloy B after anodizing and sealing was 44 percent; whereas, the reflectivity of Alloy A was only 34 percent, with a silver mirror standard being rated at 90 percent in both cases.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. An aluminum alloy having improved oxidation resistance at high temperature consisting essentially of 0.5 to 3 percent magnesium, 0.02 percent to 0.5 perstantially dissolved in solid solution in the matrix. cent silver, from 0.001 to 0.2 percent iron, from 0.001 2. An alloy according to claim 1 wherein the magto 0.l5 percent silicon, up to 0.10 percent copper, up nesium te iS rom 0.8 to 2.8 percent. to 0.10 percent manganese, up to O l percent zinc, up 3. An alloy according to claim 1 wherein the silver to 0.05 percent chromium, up to 0.05 percent titanium, 5 content from to P balance essentially aluminum, wherein the silver is sub- 

1. An aluminum alloy having improved oxidation resistance at high temperature consisting essentially of 0.5 to 3 percent magnesium, 0.02 percent to 0.5 percent silver, from 0.001 to 0.2 percent iron, from 0.001 to 0.15 percent silicon, up to 0.10 percent copper, up to 0.10 percent manganese, up to 0.10 percent zinc, up to 0.05 percent chromium, up to 0.05 percent titanium, balance essentially aluminum, wherein the silver is substantially dissolved in solid solution in the matrix.
 2. An alloy according to claim 1 wherein the magnesium content is from 0.8 to 2.8 percent. 